1. Field of the Invention
The present invention relates generally to mobile telecommunications, and in particular to determination of the location of a mobile station.
2. Description of the Related Art
It is often desired or necessary to be able to determine the location of a mobile station in a mobile telecommunications network. Examples are when a subscriber wishes to use location dependent services, e.g. “connect me to the closest pizza restaurant”, or when a subscriber makes an emergency call (such as 911). Especially in the latter case, it may be crucial for the operators to be able to determine the location of the mobile station, such as for example if the caller does not know the location or if he suffers a heart attack and is unable to respond at all. For these and other reasons, several methods and systems for mobile station location determination have been developed.
In U.S. Pat. No. 6,140,964, Sugiura et al. present a solution in which a neural network is used to determine the location. The neural network has an input layer that receives the radio signal strength from a number of base stations, one per input port in the input layer. The neural network learns the correlation between the signal strengths and the mobile station location and is then able to estimate the location once this correlation has been made. The system can of course fine-tune the correlation later on. Disadvantages of this solution is that the system must learn the correlation, i.e. someone must initiate the system by taking a mobile station and making measurements, and that the system is vulnerable to changes in base station placements, i.e. if a base station moves, the system has to be re-educated.
Global Positioning System (GPS) coordinates are used for positioning in many systems, either on their own or in combination with other positioning techniques as the one mentioned hereinbefore. In U.S. Pat. No. 5,724,660 Kauser et al. present a method and an apparatus wherein GPS coordinates are compared with the calculated location area to increase the accuracy of the location determination. A first disadvantage of this solution is that the mobile station must comprise a GPS receiver/processor that among other things increases the price of the mobile station. A second disadvantage is that the solution relies on a system that usually is not owned or controllable by the company that performs the location calculations which introduces a certain uncertainty.
Rudowicz, U.S. Pat. No. 6,006,067, and Grayson et al., EP-1,033,582 A1, both present solutions using signals from orbiting satellites. The signals are measured and straight calculations are performed using the results of the measurements, e.g. angle a is calculated according to a certain formula and the angle is used together with other results to give one answer to the location of the mobile station. A disadvantage of this approach is that the measurements are not free from errors, which means that there will be errors in the results, and it is impossible or difficult to know how good the estimate is.
Nguyen, U.S. Pat. No. 5,930,713 states that there are many known methods to determine mobile station position, such as for example using signal strength or timing advance measurements made by the mobile station, but Nguyen fails to disclose any details about these methods. Similarly, Dufour, U.S. Pat. No. 5,613,205, uses signal strength measurements to calculate the distance from each base station and calculates the intersection point of the resulting arcs. However, Dufour does not explicitly show the calculations behind the distance determination, nor does he address some problems, such as bad data as will be shown hereinafter, in the calculation of the intersection points.
Messier, U.S. Pat. No. 6,246,861, uses a channel model equation with a path loss exponent to determine the distance between a base station and the mobile station. Problems with this solution are that the channel model may be inferior to the free space propagation model, that the path loss exponent usually varies with the distance, and that the resulting equations need to be solved for distance and path loss exponent. In many cases, a method with a path loss exponent makes the calculations more complex than necessary.
It can thus be appreciated that it would be advantageous to have solution for mobile station location determination that overcomes disadvantages of the prior art. This invention provides such a solution.